The present invention relates to an improved process for the preparation of polyisocyanates with a biuret structure by continuous reaction of excess amounts of organic diisocyanates having exclusively aliphatically and/or cycloaliphatically bonded isocyanate groups with organic diamines having exclusively aliphatically and/or cycloaliphatically bonded primary amino groups at elevated temperatures by 2-stage addition of the isocyanate component. Polyisocyanates prepared in this way are distinguished by a high stability, can be prepared with a high space/time yield and have a low content of by-products. In a given production installation, process variants for obtaining biuret polyisocyanates with required target parameters (viscosity, NCO content) are possible in a simple manner.
The preparation of aliphatic polyisocyanates with biuret structures has been known since 1958, (DE-A 1101394). Further possible preparation processes are described in a review article (Laas et al., J. prakt. Chem. 336, 1994, 185-200) which discusses the advantages and disadvantages of the particular processes.
A distinction is made in principle between two process groups: on the one hand the so-called water processes, in which the diisocyanates are reacted with excess amounts of water to give ureas, and these are subsequently reacted with excess amounts of diisocyanates to give biurets, and on the other hand the so-called diisocyanate/diamine processes, in which urea is prepared directly from isocyanate and a deficient amount of amine and the biuret is subsequently prepared in turn with an excess of diisocyanate. As stated in the review article cited above (Laas et al.), numerous variants have been developed and described for both processes. In the processes described, predominantly hexamethylene-diisocyanate (HDI) and optionally hexamethylenediamine (HDA) are used for the preparation of industrially the most important HDI biurets, the biurets initially obtained, which are present as a solution in excess diisocyanate, being freed from the excess diisocyanate by distillation and/or extraction and isolated as low-monomer biuret polyisocyanates.
Biuret polyisocyanates prepared by the water process are as a rule distinguished by a good monomer stability, i.e. stability towards partial cleavage back into free diisocyanates, good dilutabilities, i.e. stability of dilute solutions with respect to clouding and precipitates under the action of moisture, and outstanding colour numbers because of the relatively mild conditions during the preparation. In the biuretization reactions by the water process, however, due to the principle of the process some of the isocyanate groups contained in the starting mixture are always converted intermediately into amino groups by reaction with a biuretizing agent. Since the isocyanate groups used up in this way have originally once been prepared by phosgenation of amino groups, this procedure seems expensive and not very economical. Furthermore, gaseous or liquid by-products are formed in these known processes, such as, for example, carbon dioxide, carbon monoxide, carbon oxysulfide, olefins or nitriles, which, with the exception of the anhydrides obtained in the pivalic acid/water process, are not recyclable and must be disposed of.
In the diisocyanate/diamine processes which have been developed, the advantage of the economical preparation manifests itself with no or only little formation of by-products, and no isocyanate groups prepared from amino groups by phosgenation are converted back into amino groups and subsequently urea groups and biuret groups. These processes have likewise been constantly developed further to a high quality level, as described e.g. in EP-A 277353. It was possible for a reduced monomer and dilution stability to be improved further by further optimizations, as described in EP-B 1158013.
As described in the literature available, HDI is preheated to temperatures of approx. 230-250° C. and reacted with HDA in a mixing chamber. During this procedure, the temperature increases further to values of typically 270-280° C. Thereafter, the temperature is cooled in stages as rapidly as possible to e.g. 180° C. In order to avoid an unnecessarily high degree of pre-damage to the heat-sensitive HDI, the educt is heated up and then overheated in the shortest possible time. In principle, however, it is not possible to rule out heat damage completely.
In order to ensure a fast reaction and biuret formation, specific mixing chamber/nozzle systems are employed for optimum and fast mixing of the isocyanate component and amine component. For flow reasons, these systems are designed for a narrow mass flow range and are therefore limited in variation. This optimum range is departed from during load changes and product change-over. The non-ideal mixing leads to a delayed and poorer biuret formation and larger urea crystals, with the consequence that the total reaction time is prolonged and side reactions increase.
From environmental aspects, heating of the large mass flows of HDI and HDA to high temperatures with the likewise necessary subsequent rapid cooling leads to a high energy consumption with a high potential of carbon dioxide emission.
The object of the present invention was therefore to obtain polyisocyanates with a biuret structure by the economically favourable diisocyanate/diamine process without exposing the diisocyanate for too long at too high a temperature, and to operate the preparation always with the optimum mixing conditions from the mixing elements during load or product change-overs. The object of the invention was in particular also to lower the high energy consumption for heating the diisocyanate stream compared with the normal process, without eliminating the proven advantages of the diisocyanate/diamine process.